A) Field of the Invention
The present invention relates to a color image pickup apparatus and more particularly to a color image pickup apparatus capable of obtaining a good white balance independently from the kinds of illuminating light sources.
B) Description of the Related Art
Technical advance of digital still cameras and digital video cameras (hereinafter collectively called “digital camera”) is remarkable. Most of digital cameras are equipped with an auto white balance (AWB) mechanism which distinguishes between different light sources and automatically adjusts a white balance.
Some digital cameras cannot correctly distinguish between different light sources. If distinguishment is impossible between sun light and fluorescent light, a photographed image has a large white imbalance.
According to prior art light source distinguishing technique, a primary color digital camera calculates R/G and B/G ratios of each pixel or a predetermined divided area of a photographing scene. The illuminating light source is estimated from a two-dimensional distribution of R/G and B/G ratios.
Estimation of an illuminating light source is made on the assumption that an average of a general photographing scene inclusive of a variety of photographing objects is gray, or on the assumption that an area of a photographing scene having a highest illumination is white.
In practice, however, there are many scenes to which the above-described assumptions cannot be applied. For example, a semi-dark scene full of green leaves under sun light is similar to a scene under a standard white fluorescent lamp, a daylight fluorescent lamp or a neutral white fluorescent lamp so that a light source distinguishing error is likely to occur. A high color rendering fluorescent lamp has the characteristics similar to those of sun light so that distinguishing between them is unnecessary. A bulb type fluorescent lamp can be easily distinguished by a prior art method.
Illuminating light sources to be paid attention are therefore a standard white fluorescent lamp, a three-wavelength daylight fluorescent lamp and a three-wavelength neutral white fluorescent lamp. In this specification, these fluorescent lamps are simply called fluorescent lamps unless otherwise specifically mentioned.
Recent digital cameras are of high quality and easy to use and are prevailing in general users in place of film cameras. There are many chances of taking scenes under various illuminating light sources with digital cameras. Good photographing is desired not only in sun light and tungsten light (incandescent lamp) but also in various kinds of fluorescent light.
Since a solid image pickup device of a digital camera has a narrow dynamic range, it is important to have a good white balance. If the white balance is poor, it is difficult to adjust the color of an image after it is taken. The auto white balance (AWB) function is therefore important because it automatically distinguishes the kind of an illuminating light source. It is not practical if a photographer manually sets a white balance by distinguishing the kind of each light source. The AWB function of a digital camera is essential in order to enjoy the usability compatible with a conventional silver salt compact camera.
It is important to improve a precision of an AWB function of a digital camera. Tungsten light can be distinguished by a conventional method at a high precision. It is desired to distinguish fluorescent light at a high precision.